Fluoranthene is a type of polycyclic aromatic hydrocarbons (PAHs), which hardly degrades in natural environment, lasts for a long period of time and exhibits carcinogenicity, mutagenicity and teratogenicity. Hence detection of PAHs is of great concern and many countries have listed PAHs as one of the priority pollutants. PAHs in water mainly come from atmospheric deposition, urban sewage, discharges of industrial wastewater, and oil leakage etc. The PAHs polluted water eventually enters into surface water and groundwater causing contamination of drinking water source. Since PAHs is hydrophobic and highly un-degradable, it usually exists in water at a low concentration. As a result, detection of PAHs is a complex and important measurement analysis issue. Among different PAHs, fluoranthene has higher fluorescence quantum efficiency and thus can be detected using fluorescence methods.
The fluorescence spectroscopy of fluoranthene requires a suitable excitation source in order to obtain strong fluorescence intensity for signal detection. The Anhui Institute of Optics and Fine Mechanics of Chinese Academy of Sciences identified two relative strong fluorescence emission zones of fluoranthene utilizing F-7000 type fluorescence spectrophotometer. One of the emission peak of the fluorescence intensity is λex/λem=286/462 nm (as shown in FIG. 1), where λex and λem denote the wavelengths of excitation and emission light, respectively. Therefore, the wavelength of the excitation light source is preferably to be 286 nm, or any neighbouring values.
The simplest way to obtain an excitation light source with such excitation wavelength is to apply a spectrometer or spectrophotometer, which utilizes spectral elements such as prisms or gratings to split a continuous spectrum light source in order to isolate an excitation light with desired wavelength. This method is simple and direct, but the disadvantage is that the intensity of the output light is very low. Also the above method generally requires sampling and testing of the spectrometer or spectrophotometer, which is not convenience while the device is in operation, which limits its use.
Tunable laser source is another alternative. Tunable laser sources have a certain wavelength tuning range and the intensity of the output laser can usually fulfil the test requirements. However, the cost of the tunable laser source is high, which costs several millions Renminbi, and thus is not popular. Moreover, if a tunable dye laser source is used, the carcinogenic dye in the tunable dye laser source will be a threat against the health of the users.
The United States Environmental Protection Agency utilized a laser-fluorescence embedded cone penetrometer system mounted on a vehicle to detect the fluoranthene content in soil. The excitation light for inducing fluorescence of fluoranthene of the above mentioned system is generated by emitting a Nd:YAG fourth harmonic laser through a Raman cell filled with a gas mixture comprising methane (CH4) and hydrogen (H2) gas. As a result, the first stokes radiation of CH4 at 288.4 nm is produced, and it is stable and closed to the excitation peak 286 nm of the preferred excitation light for fluorescence detection of fluoranthene. But the drawback is that CH4 decomposes easily due to the long stimulated time of CH4 by the Nd:YAG fourth harmonic laser
            CH      4        ⁢          ⟶      laser        ⁢    C    +      2    ⁢          H      2      (The Anhui Institute of Optics and Fine Mechanics of Chinese Academy of Sciences has conducted similar experiment and reported this phenomena with detailed analysis). As a result, micro-carbon granules (with diameters of tens to hundreds of microns) will be generated, which will attach to the glass windows at both ends of the Raman cell and reduce the transmittance of the Raman cell. Moreover, the decomposition process reduces the CH4 content in the Raman cell which in turn reduces the intensity of the output of the first stokes radiation (288.4 nm).
Therefore there is a need to develop a low-cost, yet stable laser source with suitable wavelength.